Control systems transmit I/O signals to and from field devices and controllers to regulate and control industrial processes. Field devices include sensors and actuators. Sensors transmit input signals representing the state of process variables to the controllers, and actuators receive output signals from the controllers and take action to affect process variables. Controllers may be connected directly to field devices and have fixed I/O capabilities, or may use I/O cards to provide additional capacity. Some controllers connect to remote I/O racks that communicate I/O signals with the controller over a digital bus or network.
I/O signals may be analog signals or digital signals. Analog I/O signals are variable, in which a voltage or current represents the magnitude of a process parameter such as flow rate or the desired position of a valve. Digital I/O signals represent one of two states: “on/off”, “opened/closed”, and the like.
A second type of digital output signal used in process control is used to actuate a relay. Many control tasks require relays that connect field devices such as actuators or motors to power sources for operation of the actuators or motors. The digital output signal opens or closes the relay to power off or power on the actuator or motor. The “Generation 4” or “G4” digital I/O line sold by Opto 22, Temecula, Calif., USA, for example, markets separate digital input modules and digital output modules for operating relays.
Sophisticated industrial processes may use a number of field devices and controllers distributed throughout the plant for process control. I/O wiring from the field devices is often brought to a central location for ease of management. The field wiring may extend to a terminal block contained in a wiring cabinet, and from the terminal block extend to the controllers.
Sensors and other field devices often require continuous power for operation. A field device may be powered through the I/O signal line or may receive power through a separate power line. Substantial engineering effort is required to define the power needs of the field devices and to arrange power distribution to the field devices during the initial planning stages of a control system. As field devices are replaced or updated, the power and power distribution requirements of the control system may also change. Additional engineering effort is required to respond to these changes.
During the initial commissioning of a control system, extensive testing of I/O signal quality may be required to assure proper installation and operation of the control system.
I/O signals may require signal conditioning for proper operation of the control system. I/O wiring from the field devices can be brought to modular signal conditioners housed in the cabinet, and from the signal conditioners to the controllers. An example of signal conditioners include the “MINI Analog Pro” line available from Phoenix Contact GmbH & Co., Blomberg, Germany. Changing a signal conditioner requires disconnecting the I/O wiring from the old signal conditioner and reconnecting the wiring to the new signal conditioner.
There is a need for an improved I/O signal conditioning system that can be interposed between field devices and controller that better addresses the diagnostic, power, and connection needs of a control system.